why mysql cann't using index to filter column after range condition ;

mysql 5.1.40
os:rhel 5.4 
engine=innodb
innodb_file_per_table


# create table 
use test; 
drop table index_test; 
create table index_test
(id  bigint(20) unsigned NOT NULL AUTO_INCREMENT ,
col1 varchar(255),
col2 varchar(255),
col3 datetime,
col4 varchar(255),
primary key (id),
key (col1,col3,col2)
) engine=innodb charset=utf8;


# Insert data 
for c1 in $(seq -w 500 )
do
 col1="username"$c1
  for  c2  in $(seq -w 500 ) 
  do
    col2="othercol"$c2
    col3=$(date +"%Y-%m-%d %H:%M:%S" -d "${c1} hour ago")
    mysql -uroot -h127.0.0.1 test -e  "insert into index_test values (null,'${col1}','${col2}','2010-06-03 00:00:00' - interval $c2 hour,'${col1}${col2}'); commit;"
  done
done


###--- test.sql1
================================================
reset query cache;
show session status like 'Innodb_buffer_pool_read_requests'; 
select count(*) from index_test 
where col1='username014'
and col3 >= '2010-05-01 00:00:00'
and col3 <= '2010-06-03 00:00:00' ;
show session status like 'Innodb_buffer_pool_read_requests'; 

## result  : 500 
## Innodb_buffer_pool_read_requests: 77 
## well done , all search in index ;
+----+-------------+------------+-------+---------------+------+---------+------+------+--------------------------+
| id | select_type | table      | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |
+----+-------------+------------+-------+---------------+------+---------+------+------+--------------------------+
|  1 | SIMPLE      | index_test | range | col1          | col1 | 777     | NULL |  656 | Using where; Using index |
+----+-------------+------------+-------+---------------+------+---------+------+------+--------------------------+

root@127.0.0.1 : test 16:25:52> show profile for query 1; 
+--------------------------------+----------+
| Status                         | Duration |
+--------------------------------+----------+
| starting                       | 0.000023 |
| checking query cache for query | 0.000068 |
| Opening tables                 | 0.000025 |
| System lock                    | 0.000004 |
| Table lock                     | 0.000024 |
| init                           | 0.000035 |
| optimizing                     | 0.000014 |
| statistics                     | 0.000083 |
| preparing                      | 0.000013 |
| executing                      | 0.000005 |
| Sending data                   | 0.000655 |
| end                            | 0.000004 |
| query end                      | 0.000002 |
| freeing items                  | 0.000037 |
| storing result in query cache  | 0.000004 |
| logging slow query             | 0.000001 |
| cleaning up                    | 0.000002 |
+--------------------------------+----------+



###--- test.sql2
================================================
reset query cache;
show session status like 'Innodb_buffer_pool_read_requests'; 

select count(*) from index_test 
where col1='username014'
and col3 >= '2010-05-01 00:00:00'
and col3 <= '2010-06-03 00:00:00'
#### 
and col2 = 'dsdlfkj'  ;
show session status like 'Innodb_buffer_pool_read_requests'; 


## result  : 0 
## Innodb_buffer_pool_read_requests: 77 
## well done , all search in index ;
+----+-------------+------------+-------+---------------+------+---------+------+------+--------------------------+
| id | select_type | table      | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |
+----+-------------+------------+-------+---------------+------+---------+------+------+--------------------------+
|  1 | SIMPLE      | index_test | range | col1          | col1 | 777     | NULL |  656 | Using where; Using index |
+----+-------------+------------+-------+---------------+------+---------+------+------+--------------------------+

root@127.0.0.1 : test 16:26:25> show profile for query 2;
+--------------------+----------+
| Status             | Duration |
+--------------------+----------+
| starting           | 0.000041 |
| Opening tables     | 0.000031 |
| System lock        | 0.000002 |
| Table lock         | 0.000005 |
| init               | 0.000007 |
| optimizing         | 0.000002 |
| statistics         | 0.000006 |
| preparing          | 0.000005 |
| executing          | 0.000114 |
| Sending data       | 0.000015 |
| end                | 0.000003 |
| query end          | 0.000001 |
| freeing items      | 0.000015 |
| removing tmp table | 0.000005 |
| closing tables     | 0.000001 |
| logging slow query | 0.000001 |
| cleaning up        | 0.000001 |
+--------------------+----------+
17 rows in set (0.00 sec)




###--- test.sql3
================================================
reset query cache;
show session status like 'Innodb_buffer_pool_read_requests'; 

select count(distinct col4 ) from index_test 
where col1='username014'
and col3 >= '2010-05-01 00:00:00'
and col3 <= '2010-06-03 00:00:00';
 
show session status like 'Innodb_buffer_pool_read_requests'; 


## result  :  500	
## Innodb_buffer_pool_read_requests:  1550
+----+-------------+------------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table      | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+------------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | index_test | range | col1          | col1 | 777     | NULL |  656 | Using where |
+----+-------------+------------+-------+---------------+------+---------+------+------+-------------+
root@127.0.0.1 : test 16:27:27> show profile for query 6;
+--------------------------------+----------+
| Status                         | Duration |
+--------------------------------+----------+
| starting                       | 0.000020 |
| checking query cache for query | 0.000071 |
| Opening tables                 | 0.000010 |
| System lock                    | 0.000002 |
| Table lock                     | 0.000016 |
| init                           | 0.000025 |
| optimizing                     | 0.000011 |
| statistics                     | 0.000060 |
| preparing                      | 0.000009 |
| executing                      | 0.000029 |
| Sending data                   | 0.002875 |
| end                            | 0.000004 |
| removing tmp table             | 0.000004 |
| end                            | 0.000001 |
| query end                      | 0.000002 |
| freeing items                  | 0.000021 |
| storing result in query cache  | 0.000003 |
| logging slow query             | 0.000001 |
| cleaning up                    | 0.000001 |
+--------------------------------+----------+


###--- test.sql4
================================================
reset query cache;
show session status like 'Innodb_buffer_pool_read_requests'; 

select count(distinct col4 ) from index_test 
where col1='username014'
and col3 >= '2010-05-01 00:00:00'
and col3 <= '2010-06-03 00:00:00'
#### 
and col2 = 'dsdlfkj'  ;
show session status like 'Innodb_buffer_pool_read_requests'; 


## result  : 0 
## Innodb_buffer_pool_read_requests:  1542
+----+-------------+------------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table      | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+------------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | index_test | range | col1          | col1 | 1545    | NULL |  656 | Using where |
+----+-------------+------------+-------+---------------+------+---------+------+------+-------------+

root@127.0.0.1 : test 16:27:58> show profile for query 8 ; 
+--------------------------------+----------+
| Status                         | Duration |
+--------------------------------+----------+
| starting                       | 0.000017 |
| checking query cache for query | 0.000048 |
| Opening tables                 | 0.000009 |
| System lock                    | 0.000003 |
| Table lock                     | 0.000017 |
| init                           | 0.000027 |
| optimizing                     | 0.000012 |
| statistics                     | 0.000065 |
| preparing                      | 0.000009 |
| executing                      | 0.000028 |
| Sending data                   | 0.001245 |
| end                            | 0.000004 |
| removing tmp table             | 0.000014 |
| end                            | 0.000003 |
| query end                      | 0.000001 |
| freeing items                  | 0.000027 |
| storing result in query cache  | 0.000004 |
| logging slow query             | 0.000001 |
| cleaning up                    | 0.000002 |
+--------------------------------+----------+
19 rows in set (0.00 sec)



## It's so Bad , col2 can't filter by index ;   
## Mysql find all record (fileted col1 , col3 ) from table , then filter the col2='dsdlfkj'.


## As we think , test.sql4 should not to researh record from table , because result=0; 
## That can be know while using index;